Simulation On Effects Of Variable Valve Lift On Working Process Of A Turbocharged Gasoline Direct Injection Engine

With the increasing emphasis on fuel economy and emissions, the researchers arestriving to develop engines having high efficiency, which can also comply with thestringent emission requirements. The Gasoline Direct Injection (GDI) engine hasreceived a wide attention because of its potential of improving combustion efficiencyand reducing emission。The new generation of GDI engines with stoichiometriccombustion has large pumping loss because of its throttle load control. In the meantime, VVL(Variable Valve Lift) can improve the fuel economy by reducing thepumping loss. In this study, simulation on the in-cylinder flow, mixture formationprocess, combustion and emissions of a turbocharged GDI engine with VVL wascarried out, aiming at understanding the engine performance by no-throttle loadcontrol.This research focused on the effect of VVL on turbocharged GDI engine. Whenthe intake valve timing, intake valve peak timing and intake valve duration were fixed,3-D simulation was carried out to study the effect of VVL on working progress of aturbocharged GDI engine, as well as the variation of in-cylinder flow, combustionprocess and emissions caused by VVL at2000r/min.The results showed that there were strong tumble motions produced in intakestroke. At the beginning of intake stroke, two reverse tumble structures emerged underthe intake valve, and as the maximum intake valve lift (MVL) reduced, the left tumblemoved toward to the exhaust valve. In the middle of intake stroke, acounter-clockwise tumble structure at the condition of high MVL, two reverse tumblestructures for middle MVL (MVL=4mm) and a clockwise tumble structure for lowMVL (MVL=1mm) can be observed. The swirl motions were very weak, and therewas no obvious swirl produced in cylinder, as the average swirl ratios were below0.1.Under low MVL condition, bigger average swirl ratio than high MVL condition wasobtained, and the maximum of average swirl ratio was up to0.1056for MVL1mm. Atthe end of compression stroke, the average swirl ratio of MVL1mm was twice as highas that of MVL7.7mm.When the speed of engine was2000rpm, VVL had a small effect on the air intakemass for high MVLs (MVL>3mm), while the small change of MVL will cause large change in air intake mass for low MVLs (MVL<3mm).Compared to throttle load control, VVL load control can optimize the mixtureformation process. Inhomogeneity of mixture was affected by MVL, it got lowerwhen MVL got lower. At ignition timing, the inhomogeneity of mixture was0.672forMVL7.7mm; while the inhomogeneity of mixture was0.622for MVL4mm.For2000rpm and low load condition, VVL load control can reduce the bumpingloss, but the inhomogeneity of mixture was increased, and the inhomogeneity ofmixture was0.56at ignition time. The burning rate was lower and the burningduration was longer than throttle load control condition, and the NOx emission wasreduced.